Tension leg marine platforms for use in the offshore oil industry are known and a connector apparatus for connecting such a tension leg to a subsea foundation is described in U.S. Pat. No. 4,320,993 (Conoco). An anchoring device that was used in such a structure in the Hutton field of the North Sea is described in U.S. Pat. No. Re. 32,274 of U.S. Pat. No. 4,459,933.
The tether line described in the Reissue specification and used in practice employed a tether of relatively small diameter and having a relatively thick wall. It has now been realised that it may be desirable to use tendons of larger diameter and with thinner walls but it is difficult to design and make a transition section that will couple such a relatively large tendon to a relatively small flexjoint such as is described in the Reissue specification, particularly where high loads are to be employed.